Since World War II, radar transponders have been used for various military and civilian purposes such as facilitating the radar tracking of aircraft at long ranges. Most transponder designs have provided for receiving radar pulses only at a preset or mechanically tuned radar frequency, and nearly all have responded at a fixed, prearranged frequency, which has generally been different from the frequency of the received pulse, although it might have been the same as the received pulse if a sufficient delay in responding were provided.
With the introduction of radars having frequency agility, transponders have been designed that utilize broadband microwave amplifiers and microwave delay lines and therefore return a signal at the same frequency as that received, but at a later time. However, to avoid undesirable oscillations due to coupling of the transponder output signal back to the input before a given pulse is completed, a microwave delay line with a delay of one pulse width or greater is needed and this is generally a large unwieldy component.